Title: METHOD AND APPARATUS FOR INTEGRATED WIRELESS COMMUNICATIONS IN PRIVATE AND PUBLIC NETWORK ENVIRONMENTS;
Inventors: Ian Leslie Sayers, Paul Jan Long, Sheausong Yang;
SC/Ser.No.: 09/188,856;
Title: METHOD AND APPARATUS FOR CONTROLLING WIRELESS NETWORKS
Inventors: Ian Leslie Sayers, Paul Jan Long, Peter K. Cripps
SC/Ser.No.: 09/271,596;
Atty Doc. No.: JETC1001DEL.
The present invention is a method and apparatus that provides for the synchronizing of clock signals in wireless networks.
Conventional Cellular Systems
Present day cellular mobile telephone systems provide for a large and increasing demand for mobile services. Cellular systems xe2x80x9creusexe2x80x9d frequency within a group of cells to provide wireless two-way radio frequency (RF) communication to large numbers of users. Each cell covers a small geographic area and collectively a group of adjacent cells covers a larger geographic region. Each cell has a fraction of the total amount of RF spectrum available to support cellular users. Cells are of different sizes (for example, macro-cell or micro-cell) and are generally fixed in capacity. The actual shapes and sizes of cells are complex functions of the terrain, the man-made environment, the quality of communication and the user capacity required. Cells are connected to each other via land lines or microwave links and to the public-switched telephone network (PSTN) through telephone switches that are adapted for mobile communication. The switches provide for the hand-off of users from cell to cell and thus typically from frequency to frequency as mobile users move between cells.
In conventional cellular systems, each cell has a base station with RF transmitters and RF receivers co-sited for transmitting and receiving communications to and from cellular users in the cell. The base station employs forward RF frequency bands (carriers) to transmit forward channel communications to users and employs reverse RF carriers to receive reverse channel communications from users in the cell.
The forward and reverse channel communications use separate frequency bands so that simultaneous transmissions in both directions are possible. This operation is referred to as frequency division duplex (FDD) signaling. In time division duplex (TDD) signaling, the forward and reverse channels take turns using the same frequency band.
The base station in addition to providing RF connectivity to users also provides connectivity to a Mobile Services Switching Center (MSC). In a typical cellular system, one or more MSCs will be used over the covered region. Each MSC can service a number of base stations and associated cells in the cellular system and supports switching operations for routing calls between other systems (such as the PSTN) and the cellular system or for routing calls within the cellular system.
Base stations are typically controlled from the MSC by means of a Base Station Controller (BSC). The BSC assigns RF carriers to support calls, coordinates the handoff of mobile users between base stations, and monitors and reports on the status of base stations. The number of base stations controlled by a single MSC depends upon the traffic at each base station, the cost of interconnection between the MSC and the base stations, the topology of the service area and other similar factors.
A handoff between base stations occurs, for example, when a mobile user travels from a first cell to an adjacent second cell. Handoffs also occur to relieve the load on a base station that has exhausted its traffic-carrying capacity or where poor quality communication is occurring. The handoff is a communication transfer for a particular user from the base station for the first cell to the base station for the second cell. During the handoff in conventional cellular systems, there may be a transfer period of time during which the forward and reverse communications to the mobile user are severed with the base station for the first cell and are not established with the second cell.
Conventional cellular implementations employ one of several techniques to reuse RF bandwidth from cell to cell over the cellular domain. The power received from a radio signal diminishes as the distance between transmitter and receiver increases. Conventional frequency reuse techniques rely upon power fading to implement reuse plans. In a frequency division multiple access (FDMA) system, a communications channel consists of an assigned particular frequency and bandwidth (carrier) for continuous transmission. If a carrier is in use in a given cell, it can only be reused in cells sufficiently separated from the given cell so that the reuse site signals do not significantly interfere with the carrier in the given cell. The determination of how far away reuse sites must be and of what constitutes significant interference are implementation-specific details. The cellular Advanced Mobile Phone System (AMPS) currently in use in the United States employs FDMA communications between base stations and mobile cellular telephones.
In time division multiple access (TDMA) systems, multiple channels are defined using the same carrier. The separate channels each transmit discontinuously in bursts which are timed so as not to interfere with the other channels on that carrier. Typically, TDMA implementations also employ FDMA techniques. Carriers are reused from cell to cell in an FDMA scheme, and on each carrier, several channels are defined using TDMA methods. The Global System for Mobile Communications (GSM), PCS1900, IS-136, and PDC standards are examples of TDMA methods in current use.
The present specification uses a GSM system for purposes of explanation but the present invention applies to any wireless system protocol.
GSM Cellular Systems
The GSM system architecture is described, for example, in detail by M. Mouly and M.-B. Pautet, The GSM System for Mobile Communications, 1992 and Mouly and M.-B. Pautet, GSM Protocol Architecture: Radio Sub-system Signaling, IEEE 41st Vehicular Technology Conference, 1991. The following sections highlight some unique aspects of GSM systems.
The GSM system specifications incorporate many advanced services and features including:
ISDN compatibility based upon Q.931
World-wide roaming with other GSM networks
Two way messaging
Data Services
FAX Services
ISDN Supplementary Services.
However, the GSM system is designed fundamentally for use in a traditional Circuit Switched environment that uses 64 kbps voice and data transport.
GSM System Architecture
The standard GSM network includes three major components, namely, the Mobile Station (MS), Base Station Sub-System (BSS) and the Network Sub-System (NSS). The GSM Specifications define the network entities and their associated interfaces within the Public Land Mobile Network (PLMN). The complete suite of specifications also includes documents that define the type approval procedures for mobile stations allowing mobile stations to be used in different countries, independently of the country in which they were type approved.
Base Station Subsystem (BSS)
The Base Station Subsystem (BSS) is composed of two main parts, the Base Transceiver Station (BTS) and the Base Station Controller (BSC). The BTS includes the radio transceivers that define the radio cell boundary and handles the radio (Um) interface protocols with the mobile station. There are a number of different cell types, macro, micro and pico, that can be deployed dependent on the terrain, subscriber density and coverage requirements. The macro cell is intended for large cell sizes with ranges from 2 km to 70 km. The micro cell is intended to provide cell sizes from 100 m to 5 km, either as an in fill or in areas serving a high density of subscribers. The pico cells are intended to support cell sizes in the range 50 m to 1 km and will be used to provide high quality local radio coverage. The BTS supports all the required channel coding, encryption and speech coding required by the radio interface. The speech transcoding may be performed locally at the BTS or remotely at the BSC or MSC. If remote transcoding is used, then the BTS is still required to control this function.
The Base Station Controller (BSC) manages the radio resources of one or more BTSs across the Abis interface. The BSC controls most of the features of the radio network, including allocation of radio time slots to a mobile station, release of the resources, interpretation of measurement results and control of radio interface hand-overs. The BSC interfaces to the NSS via the A-interface to the MSC.
Radio Transmission
The BTS is responsible for maintaining the radio link with the GSM Mobile station. Currently the GSM system can support three frequency bands at 900, 1800 and 1900 MHz. However in each band the physical TDMA structure is identical. Each RF carrier is divided into eight time slots using TDMA. Groups of eight consecutive time slots form TDMA frames.
There are two types of logical channels that are sent over the physical radio interface and these are Traffic channels and Common Control Channels. The traffic channels provide a bi-directional point-to-point transmission link to a mobile station. Full-rate Traffic Channels (TCH/F) and half-rate Traffic Channels (TCH/H) are allocated together with a low bit-rate Slow Associated Control Channel (SACCH), which typically transmits measurements needed for hand-over decisions. There are also eighth-rate Traffic Channels, also called Stand-alone Dedicated Control Channels (SDCCH), which are used primarily for transmitting location updating information. In addition, a TCH slot can be pre-emptied for signaling, in which case it is called a Fast Associated Control Channel (FACCH), which can be either full-rate or half-rate TCHs.
Common channels can be accessed both by idle mode mobiles, in order to change to dedicated mode, and by dedicated mode mobiles, to monitor surrounding base stations for hand-over information. The common channels, which are defined include:
Broadcast Control Channel (BCCH) Continually broadcasts, on the downlink, information including base station identity, frequency allocations, and frequency-hopping sequences.
Frequency Correction Channel (FCCH) and Synchronization Channel (SCH) Used to synchronize the mobile to the time slot structure of a cell by defining the beginning of a TDMA frame.
Random Access Channel (RACH)
Slotted Aloha channel used by the mobile to request access to the network.
Paging Channel (PCH)
Used to alert the mobile station of incoming call.
Access Grant Channel (AGCH)
Used to allocate an SDCCH to a mobile for signaling (in order to obtain a dedicated channel), following a request on the RACH.
Speech and Channel Coding on the Radio Interface
Speech in GSM is digitally coded at a rate of 13 kbps, so-called fill-rate speech coding. This rate is efficient compared with the standard ISDN rate of 64 kbps. In addition, GSM also supports a half-rate speech code operating at around 7 kbps, effectively doubling the capacity of a network.
This 13 kbps digital stream is split into (260 bits every 20 ms). This data contains some forward error correction raising the gross bit rate after channel coding to 22.8 kbps (or 456 bits every 20 ms). These 456 bits are divided into eight 57-bit blocks, and the result is interleaved amongst eight successive time slot bursts for protection against burst transmission errors.
Each time slot burst is 156.25 bits and contains two 57-bit blocks, and a 26-bit training sequence used for equalization. A burst is transmitted in 0.577 us for a total bit rate of 270.8 kbps, and is modulated using Gaussian Minimum Shift Keying (GMSK) onto a 200 kHz carrier frequency. The 26-bit training sequence (TSC) is of a known pattern that is compared with the received pattern to perform a channel estimation. This channel estimation is then used to recover the received signal. Forward error control and equalization contribute to the robustness of GSM radio signals against interference and multipath fading.
Network Subsystem
An essential component of the Network Subsystem is the Mobile services Switching Center (MSC). The MSC provides the functions required to switch calls to/from the mobile user and the PSTN or ISDN fixed network. In addition the MSC also provides the functions needed to track and maintain communication with a mobile subscriber, these include registration, authentication, location updating, inter-MSC hand-overs, and call routing to a roaming subscriber. In order to adequately maintain contact with the network subscribers the GSM PLMN employs a number of databases. The main database functions are provided by two Location Registers, known as the Home location Register (HLR) and Visitor Locations Register (VLR).
The Home Location Register (HLR) contains all the information related to an operators subscriber database. The HLR is the main database for a network. The HLR stores both static and dynamic data related to the subscriber. Static data includes items such as International Mobile Subscriber Identity, subscriber MSISDN number and registered supplementary services. Dynamic data includes, for example, current location of the mobile user, in terms of VLR and MSC E.164 Number, and call forwarding numbers. The HLR downloads the required data to a VLR database when a Mobile User registers in a VLR area, it also provides the necessary functionality to terminate mobile calls.
The Visitor Location Register (VLR) stores the subscribers data, downloaded from the HLR, for mobile stations currently located in the VLRs area. The data stored in the VLR may include information from the Home HLR and foreign HLRs. The VLR is used to provide both Mobile Originated and Mobile Terminated call functionality. The VLR is defined as an independent database in GSM, however in order to optimize system performance many implementations combine MSC and VLR functionality, this effectively makes the MSC and VLR areas identical.
The remaining two databases are associated with security aspects of the network. The Authentication Center (AUC) is a secure database used to provide authentication keys, based upon a secret key (ki), to the HLR and subsequently the VLR for verifying the validity of the users subscription. The algorithm (A3) used to perform the authentication of the subscriber is stored in the users Subscriber Identity Module card and Authentication Center (AUC), only the challenge and result are sent on the radio interface. The challenge is also used by another algorithm (A8) to generate the key required by the A5 radio interface encryption algorithm. Although GSM defines possible A3 and A8 algorithms they are more realistically defined by the operator. The remaining database is the Equipment Identity Register (EIR) which contains a list of valid International Mobile Equipment Identity (IMEI) values. The database can therefore be used to control the use of stolen, non-type approved or faulty mobile equipment. When a mobile subscriber registers with the network the IMEI can be obtained and validated against the EIR data. If the IMEI is blacklisted, then action can be taken to prevent network access by the user.
Operations and Maintenance
Associated with the BSS and NSS equipment are Operations and Maintenance Center, OMC-R and OMC-S, respectively. The OMC-R provides the operations and maintenance control of the GSM BSS functions. The OMC-R is used to perform the following functions:
Configuration of the Cells, this includes allocation of radio frequency, hand-over parameters, cell parameters and timer values.
Performance monitoring. This function allows the OMC-R to receive statistical information about the various aspects of the BSS, such as number of calls, numbers of hand-overs etc.
Alarm reporting. The OMC-R is used to view and handle various alarms that are originated by the BSS. These may include hardware or software failures, loss of connections, etc
Software Download. The OMC-R is responsible for providing and updating the software load to the BSS.
The NSS equipment is associated with the OMC-S. The OMC-S provides the same type of high level functions as the OMC-R. In addition the OMC-S may be used to provide user data administration for the HLR and VLR. However this function is more usually provided by a dedicated Administration Center which can also deal with Billing Server requirements and SIM data.
Services Provided by GSM
GSM was designed with ISDN interoperability as a pre-requisite, consequently the services provided by GSM are a subset of standard ISDN services, however this is rapidly eroding as more ISDN services are developed within the GSM format. The GSM system provides a range of Basic and Supplementary Services. The Basic Services are further sub-divided into Teleservices and Bearer Services.
The Teleservices include:
Speech, the most basic service
Short Message, a two way messaging service
Group 3 FAX, this services allows connection to Group 3 FAX machines
Cell Broadcast, this service allows messages to be broadcast to the mobile stations.
The Bearer Services include:
Asynchronous Data 300-14400 bps, allows access to normal V-Series Modems
Synchronous Data 300-14400 bps, allows access to CSPDNs
PAD Services
Packet Services
The Supplementary Services are intended to enhance the functionality of the Basic Services. The Phase 1 specifications only provide Call Forwarding and Call Barring Services. The Phase 2 Supplementary services included Line identification services, advice of charge, multi-party, call waiting and call hold. The Phase 2+ services will include Call Transfer, Call Completion Busy Subscriber (CCBS) and Optimal routing capabilities independent as possible from the underlying specifics of the mobile network. Another sublayer is Supplementary Services, which manages the implementation of the various supplementary services, and also allows users to access and modify their service subscription. The final sublayer is the Short Message Service layer, which handles the routing and delivery of short messages, both from and to the mobile subscriber.
Problems with Clock Synchronization
In wireless systems, it is important to maintain frequency accuracy and network wide synchronization at BTSs. In the prior art, synchronization of BTSs has been achieved by either free-running accurate crystal oscillators or complex timing operations that determine and compensate for distances between the BSCs and attached BTSs. Both of these schemes present significant problems. In the case of the free-running oscillator, the maintenance costs and long term stability cause problems and in the case of BSC-BTS time measurement, the BSC and BTS need be from the same vendor and all the BTSs to be synchronized need to be attached to the same BSC. Consequently both of these schemes tend to be inadequate for large scale private systems which include indoor deployments. The use of conventional GPS receivers is not adequate since it is not always possible to receive the required GPS radio signals, particularly when indoors. Additionally, in those instances where the GPS source suddenly fails or disappears due to a failed antenna system or other system failures, it is necessary to reacquire the synchronization source through other means.
In accordance with the above background, it is an object of the present invention to provide wireless systems that facilitate reliable and accurate synchronization of clock signals using master clock signals from a global positioning system (GPS) or other accurate frequency source in a manner that facilitates both indoor and outdoor operations.
The present invention is used in a communication system that extends over a cellular region formed of a plurality of wireless cells where each cell covers a portion of the cellular region. Each cell has a base station a clock unit that facilitates reliable and accurate synchronization with other clocks units in other cells using a master clock signal from a global positioning system (GPS) or other accurate frequency source. The clock unit in each cell includes a local clock providing a local clock signal for clocking the cell. A master clock source provides one or more master clock signals and a clock synchronizer receives the master clock signals and forms a synchronizing master clock signal for synchronizing the local clock signal.
In one or more of the wireless cells of the communication system, the clock unit is a master clock unit which includes a clock distributor for distributing a distributed master clock signal to one or more other wireless cells having slave clock units. Typically, the master clock source includes a GPS receiver for receiving a GPS signal as a master clock signal. In another embodiment, the master clock source includes a high accuracy crystal controlled oscillator for generating an oscillator signal as a master clock signal.
In one embodiment, the base stations are connected together in a local area network that functions to help exchange clock signal information among the clock units.
The present invention is useful in a communication system formed by a private network that includes a private wireless network. The communication system also is useful in a public wireless network using a public wireless protocol, such as GSM, TDMA, or CDMA that is typically connected to other public networks, such as PSTN, ISDN and the Internet, using a wired protocol, such as IP. The private network also typically includes a local area network (LAN) and the private network typically connects to the public networks using a wired packet protocol, such as IP.
In connection with some embodiments, the public and private wireless networks operate with the same public wireless protocol, such as GSM or TDMA, and the private wireless network additionally operates with a wired packet protocol, such as IP.
The private wireless network uses private base stations (P-BTS) which include software for a wireless protocol, such as GSM or TDMA, include software for private network operation with a wired protocol, such as IP.
The communication system permits users to operate freely in both public and private wireless networks using standard mobile stations while achieving high private network data rates. The communication system typically uses normal wireless handsets or other mobile or fixed stations without need for any modifications.
The private base stations (P-BTS) in one embodiment are directly connected to a private LAN and thereby enable standard wireless stations to make and receive calls over the LAN. Also, the range of calls, using standard Internet protocols, extends between LANs and between different corporations over the Internet without requiring the support of a switch (e.g. MSC). The wireless stations can freely roam between the public wireless network and the private wireless network and a single telephone number can be assigned to a mobile station for use in both the public and the private wireless networks.
The foregoing and other objects, features and advantages of the invention will be apparent from the following detailed description in conjunction with the drawings.